The present invention relates to control devices for actuators. More particularly, the present invention relates to a control device for an actuator of construction equipment.
In conventional circuits used to control, for example, hydraulic cylinders, providing for stable and continuous operation has proven highly problematic. This difficulty is heightened by the constraints involved in high-speed engine operation. Attempts have been made to solve these problems using known circuits.
Turning now to FIG. 5, an example of one of such conventional circuits is shown. A main pump 12 is driven by an engine 11 to feed pressurized fluid through a discharge port 12 and an oil feed channel 13 to inputs of a plurality (three in the illustrated embodiment) of control valves 14a, 14b, and 14c. Directional control valve 14a is shown in schematic detail. Directional control valves 14b and 14c are identical to control valve 14a, and internal details thereof are omitted.
Directional control valves 14a, 14b, and 14c feed working fluid to actuators 15a, 15b, and 15c. The direction and volume of flow of the fluid is controlled by respective spools of control valves 14a, 14b, and 14c. Working fluid discharged from actuators 15b and 15c returns to a tank line 16, through an oil return channel and control valves 14a, 14b, and 14c.
Control valve 14a controls the feeding of pressurized fluid on upper oil feed channel 22 and lower oil feed channel 23 to an actuator 15a, a hydraulic cylinder. Actuator 15a is the target cylinder (the object to be controlled).
A pilot pump 17 is driven by engine 11 to feed pressurized fluid on a pilot pressure line 18 to a plurality of pilot valves 19a, 19b, and 19c.
Each pilot valve 19a, 19b, and 19c is controlled by its respective operating lever 20a, 20b, and 20c. Operating levers 20a, 20b, and 20c are controlled by an operator of the construction equipment.
Pilot valves 19a, 19b, and 19c control the flow of pressurized fluid from pilot pressure line 18 to pilot pressure receiving sections of respective pilot lines a1/a2, b1/b2 and c1/c2.
In its quiescent condition shown in the figure, operating lever 20a is in its neutral (unactuated) position. In this position, the spool of control valve 14a blocks the flow of pressurized fluid to and from upper and lower oil feed channels 23 and 22. A return channel 26 in control valve 14a permits return flow of fluid from a common return line 27 carrying discharge fluid from control valves 14b and 14c.
In addition to its quiescent condition, control valve 14a may be displaced into one of two operating positions. When operating lever 20a of pilot valve 19a is biased in the direction a1, the spool of control valve 14a is displaced upward by pilot pressure from pilot line a1 from its neutral position shown to a direct feed position in which metering oil channel 21 connects discharged fluid from upper oil feed channel 23 to a tank line 16, and connects fluid from check valve 29 to lower oil feed channel 22. This condition urges the piston in actuator 15a in the upward direction of moving an element (not shown) of the equipment of which the present control system is a part. In this position, common return line 27 is blocked by a return channel 26, which is closed in this position. A return-side throttle 25 restricts the flow of fluid therethrough to control the rate at which the piston of actuator 15a is capable of moving.
The second position of the spool of control valve 14a is in the downward direction. In this direction, the feeding and return flows from upper and lower oil feed channels 23 and 22 are reversed, compared to the first direction, whereby the piston of actuator 15a is moved downward. As in the first position, return flow of fluid from common return line 27 is blocked.
In the prior-art device, return-side throttle 25 is a fixed-diameter aperture whose size, and therefore whose maximum fluid flow rate, is fixed during manufacture of control valve 14a. The maximum fluid flow rate through control valve 14a is therefore determined at manufacture, and no provision exists to vary the rate at which the piston of actuator 15a moves. Although meter-in oil channel 21 is unrestricted, the flow rate therethrough is controlled by return-side throttle 25.
The maximum flow of oil that can be supplied by main pump 12 is proportional to the speed of rotation of engine 11. Stroke control of control valve 14a remains constant in relation to the angular degrees of actuation of operating lever 20a, regardless of quantity of available from main pump 12.
When engine speed is reduced, or other actuator spool or spools 14b, 14c are operated, the effect is the same as when the aperture of return-side throttle 25 is reduced because, even with control valve 14a set at full stroke the aperture of return-side throttle 25 is constant.
Under some conditions of inertial or gravity load, the aperture-limited flow of oil to actuator 15a may be insufficient to produce the required motion, or resist input forces. This may cause actuator 15a to void thereby produce temporary stoppage and generally unstable operation.
Such problems with required cylinder speed exceeding the available oil supply flow, and concomitant loss of control result in high levels of dissatisfaction among users.
Attempts have been made to simply increase the aperture size of the return-side throttle in order to eliminate the above drawbacks. This is not and adequate solution although it may prevent temporary stoppage of the system during operation. Problems of insufficient oil flow still remain during low engine speed or during simultaneous operation with other actuators.
Similarly, during high engine speed operation when actuator 15a is operated alone, large quantities of oil are restricted by return-side throttle 25, resulting in excessive heat generation.